1. Field of the Invention
This invention concerns a three-dimensional (3D) database generating system for generating a database comprised of three dimensions and a method for generating three-dimensional database therefor.
2. Description of Related Art
Conventional methods for generating a three-dimensional database that generate a database comprised of three dimensions such as a three-dimensional topographical map, operate by using an aircraft or a satellite to generate a three-dimensional database or by generating a three-dimensional database through projecting objects in 3D.
The method of generating a three-dimensional database using an aircraft will now be described.
There are three main methods used in the generation of a three-dimensional database using an aircraft. The first method involves generating a three-dimensional database using camera images taken from an aircraft.
Among the methods for generating a three-dimensional database using an aircraft, the method of using camera images to generate a three-dimensional database involves mounting a camera on an aircraft and taking overlapping two-dimensional images and subjecting the areas of the images that overlap to stereograph processing to obtain height data for the object. Stereograph processing here means manually defining height using stereoscopes or extracting height with a computer aid that registers the images with previously stored information from three station points.
With the method for generating a three-dimensional database using camera images however, there is substantial distortion in the peripheral regions of a photograph because an aircraft only flies at an altitude of 2 or 3 kilometers. If there is a building in a peripheral area of the photograph the building seemingly casts itself over adjacent space, thereby submerging a substantial portion of the photograph in its shadow. Accordingly, the system is not efficient for projecting an area over a wide spectrum in 3D. For example, if there is a building 100 meters high in a peripheral area of a photograph an area about 50 meters behind the building would be engulfed in shadow.
The method of three-dimensional database generation through synthetic aperture radar (SAR) among the methods for generating a three-dimensional database using an aircraft, involves mounting a stereograph synthetic aperture radar on an aircraft and obtaining height data for measured objects through subjecting two-way observation data to interference processes. The processes required for this method of three-dimensional database generation through synthetic aperture radar are complex however, and are therefore not implemented in practice.
Another method of three-dimensional database generation using an aircraft is the method of generating a three-dimensional database using a laser altimeter. This method involves mounting a laser on an aircraft and obtaining height data for observed objects through emitting a beam from the laser and measuring the time differential for the reflection of the beam.
An issue for this method of generating a three-dimensional database using a laser photometer however, is that technology for extracting one “chunk” of object, a building for example, from a collection of assembled height data, where height data for the object concerned can be collected such that it is detailed and highly accurate, has still not been established. Accordingly, the search is underway to find a method for specifying the outlines of a building manually based on simultaneously measured photographic images and collecting height data (DEM) object by object. Even if, for that required method, polygonization based on height data were performed, when texture is applied for the ground surface or the roof or side faces of a building or else, combining of simultaneously taken photographs and DEM data is required. Because the simultaneously taken photographs and DEM data are collected by different equipment however, their respective observation coordinate systems are different making the registering operation complex and difficult.
The method for generating a three-dimensional database using a satellite will now be described.
The method of generating a three-dimensional database using a satellite involves photographing the ground from a satellite positioned at an extremely high altitude of between 400 kilometers to 900 kilometers. The satellites used are GPS satellites and as the position and orbit of such a satellite observing the earth can be fixed with a high degree of precision it is possible to obtain images in which objects are located with a high degree of accuracy. Because the ground is photographed from a high altitude an image that covers a wide area and has minimal distortion in the periphery can be obtained.
The method for generating a three-dimensional database through projecting objects in 3D will now be described.
The methods for generating a three-dimensional database using an aircraft or satellite as described above are chiefly used for generating a three-dimensional database for the ground surface over a wide area. There is a method for generating a three-dimensional database by using similar technology and projecting a small object in 3D.
When projecting a small object in 3D the small object is placed on a revolving base and photographed from a plurality of angles to obtain a stereographic image of the object, projecting it in 3D. When projecting a large object in 3D, such as a piece of real estate, the object is photographed from a plurality of angles by changing the point from which each photograph is taken thereby obtaining a stereographic image of the object, projecting it in 3D.
Where such a method for generating a three-dimensional database by projecting the object in 3D is used, when the object is a small object that is placed on a revolving base and photographed from a plurality of different angles, something like a black curtain can be placed in the background behind the object to make the object under observation standout When the object is something large the resolution is extremely high in comparison to that obtained when an aircraft or satellite is used and because the object being photographed is easily distinguishable from the background that object is clearly differentiated and the outlines of the object can be defined easily.
All the above-mentioned existing methods for generating a three-dimensional database incur the following problems however.    (1) When the method of generating a three-dimensional database using an aircraft is used, the scope of area that can be photographed during one flight is extremely narrow so many flights must be made. This means that considerable expenses may be incurred in order to obtain sufficient data from aerial photographs required to generate the desired database.
Further an aircraft may only fly within restricted regions inside a country making it difficult to generate a global three-dimensional landscape database.    (2) The method all generating a three-dimensional database using a satellite is limited in the resolution obtainable in comparison to that obtained when generating a three-dimensional database using an aircraft. Further, weather conditions can have an impact.    (3) When generating a three-dimensional database by projecting an object in 3D it is not possible to project the area of a broad region in 3D. Further, the place from which the object under observation is photographed cannot be fixed with absolute certainty, so especially in the case of a large object, difficulties are encountered when taking photographs from a plurality of different angles.